A sewer is an underground system designed to carry away waste and stormwater from homes and streets. It helps prevent flooding and keeps communities clean by transporting sewage to treatment facilities, where it is processed before being released back into the environment. Sewers are typically made of materials like concrete, clay, or plastic and can vary in size. They are an essential part of urban infrastructure, ensuring public health and sanitation by managing the disposal of wastewater and reducing the risk of contamination in areas like rivers and lakes.